1. Field of the Invention
The present invention relates to a control device which controls a controlled variable of a multiphase electric rotating machine by controlling switching elements of a power inverter circuit through which terminals of the electric rotating machine are electrically connected with positive and negative electrodes of a direct current power source, and also relates to a method of controlling the electric rotating machine by using the control device.
2. Description of Related Art
A control device for a three-phase motor has been used. For example, to control three phase currents, actually flowing through respective phase windings of the motor, to instructed values under the feed-back control, this device performs a pulse width modulation (PWM) control, based on triangular-pulse wave comparison, for the motor. In this PWM control, instructed values of voltages to be applied from an inverter to the respective phase windings of the motor are calculated, and switching elements of the inverter are controlled based on differences between the level of a carrier signal shaped in a triangular wave and the instructed voltage values. Therefore, the phase voltages applied to the phase windings can be controlled to the instructed values, so that phase currents flowing through the phase windings can be controlled to desired values.
However, when the motor is set in a high rotation speed region so as to generate a high torque at a high rotation speed, the amplitude of the instructed phase voltage is increased so as to be equal to or higher than a half of an input voltage of the inverter. Therefore, in the high rotation speed region of the motor, it is difficult to control the voltage, actually outputted from the inverter, to the instructed value under the PWM control.
To perform the control for the motor in the high rotation speed region, the rectangular wave control has been used in place of the PWM control. In this rectangular wave control, the on-off period of each switching element of the inverter is set to be substantially the same as the rotational cycle duration of the motor denoting the period of time required for the rotation of 2π expressed by electrical angle. The on-off period is defined as the reciprocal number of the on-off repetition frequency. However, the ratio (hereinafter, called voltage utilization ratio) of a controlled voltage outputted from the inverter to a voltage inputted to the inverter becomes large in the rectangular wave control, as compared with the maximum voltage utilization ratio of 1/2 in the PWM control. The voltage utilization ratio in the PWM control is maximized when the amplitude of the controlled voltage becomes 1/2 of the voltage inputted to the inverter.
Therefore, when one of the PWM control and the rectangular wave control is switched to the other one in response to a change of the torque required of the motor, the voltage utilization ratio is discontinuously changed. In this case, the torque generated in the motor becomes unstable due to this discontinuous change.
To continuously change the voltage utilization ratio even when the operation of the motor is shifted to the high rotation speed region, Published Japanese Patent First Publication No. H09-047100 discloses a control device for a motor. In this device, d- and q-axis components of an instructed voltage defined on the dq rotational coordinates system are calculated from an instructed current, determined based on a target torque, for the current feed-back control. When the amplitude of the controlled voltage outputted from an inverter to a three-phase motor is equal to or higher than a half of the voltage inputted to the inverter, the inverter is operated according to either a phase of the controlled voltage, calculated from the instructed voltage, or one of pulse patterns of a voltage signal stored in a read only memory (ROM). Each pulse pattern is formed to be appropriate to one value of the voltage utilization ratio. Therefore, the voltage utilization ratio in the control based on the pulse patterns of the ROM can be increased so as to approach the value obtained in the rectangular wave control.
This Publication also discloses that each of the pulse patterns, used until the rectangular wave control is performed, is designed so as to minimize higher harmonic waves of the controlled voltage applied to the motor.
However, the pulse pattern disclosed in the Publication consider ably differs from the pulse pattern in the PWM control based on triangular-pulse wave comparison. In this case, when one of the pulse pattern control based on pulse patterns and the PWM control is switched to the other one to turn on and off the switching elements of the inverter under the selected control, controllability of the control device for the output voltage of the inverter is sometimes lowered. Therefore, it is required to heighten this controllability.
Further, the inverter generally has switching elements of an upper arm connected with the positive electrode of a power source and switching elements of a lower arm connected with the negative electrode of the source, and two switching elements of the arms are serially connected with each other for each phase. The two switching elements of the arms are alternately turned on and off according to a pulse pattern. In this case, the inverter easily causes a short circuit between two switching elements of the arms. To prevent this short circuit, the timing of instructing the turn-on operation to each switching element of one arm is generally delayed by a dead time from the timing of instructing the turn-off operation to the corresponding switching element of the other arm. Therefore, it is required to set the pulse width in the pulse pattern such that two switching elements serially connected with each other are simultaneously set in the off state at least during the dead time. However, when the rotational speed in the motor is increased, the off period corresponding to the distance between adjacent pulses in the pulse pattern sometimes becomes lower than a limit value required to set the two switching elements in the off state together during the dead time. In this case, the switching elements cannot be turned on and off according to the pulse pattern, and the three-phase voltage actually applied to the motor is not controlled to an instructed value. As a result, controllability of the control device for the three-phase motor is undesirably lowered.
Therefore, it is preferable to heighten this controllability while the switching elements are set in the off state together by the dead time or more before one of the switching elements is turned on.